Feature: Soldering
The right soldering atmosphere can mitigate soldering defects
By Rehm Thermal Systems’s technical team
atmosphere in soldering?” Tis question can be answered in the context of typical defect scenarios, including those involving solder balls, beading, voiding, whiskers, graping, head-in-pillow, wetting defects and tombstones. While a soldering atmosphere can cause or accelerate various soldering defects, it can also prevent or reduce them.
A
Solder balls / beading A very common defect involves solder balls or beads on two-pole components (chips), referred to as “beading”. Beading is caused when solder grains in the paste make their way under the component and are then squeezed out of the gap under the chip during reflow. All the solder pastes examined show reduced beading aſter reflow soldering in
typical question put to a manufacturer of reflow soldering systems like Rehm Termal Systems is oſten: “What is the benefit of a nitrogen
a nitrogen atmosphere. Te reason for this appears to be the better and faster fusion of the individual solder grains in the paste into a uniform solder volume. Te absence of oxygen prevents the formation of oxide films on the surface of the solder grains, which would hinder fusion. Tis reduces the probability that individual solder grains will penetrate the gap below the chip.
Voiding Similarly, the lower pore formation – otherwise known as “voiding” – in surface and QFN (quad flat no leads) soldering is attributed to better wetting. Te fast-wetting solder can expel the gases produced during reflow soldering more efficiently from its liquid volume. However, these results cannot be generalised to all solder joints.
Graping Flux properties are subject to constant change and improvement. Still, classic properties such as cold and hot slump are not to be neglected in reflow profiling. Time
36 July/August 2022
www.electronicsworld.co.uk
above liquidus and slope gradients are not the only decisive factors here; for example, in miniaturisation, the paste grain size also plays a role. Te smaller the grains, the more they have to be protected by the flux. If the protective flux has disappeared from the paste grain due to bleeding in the preheating area (hot slump), it oxidises and will subsequently remelt and no longer fuse with other paste parts. Te oxide layer is impenetrable and the appearance resembles a bunch of grapes – hence the name graping; see Figure 1. Te inert nitrogen atmosphere enlarges the process window, but does not completely eliminate the flux influence.
Head-in-pillow (HiP) Te head-in-pillow or head-on-pillow effect is caused by an oxide layer on the BGA (ball grid array) ball that prevents the paste grains from fusing. Typically, twisting and warping of the BGA and/or PCB causes separation of the paste deposit from the BGA ball; see Figure 2. Lack of flux activation causes an oxide layer to form on the ball, which later prevents it from
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52
